A casing collar locator is a device which finds or locates the collars or basing joints which join a string of well casing together. After a well has been drilled and as part of the completion procedure, the well typically is cased. Casing is assembled by joining individual joints together. They are normally joined with an external collar which threads to a pair of adjacent joints. The extra metal in the large collar is easy to magnetically locate. In recent times, the collar can be avoided by incorporation of a different type of thread construction, namely, omission of the collar for only a pin and box thread connection between adjacent joints. This reduces the mass of metal around the threaded connection. It provides a more uniform wall thickness while reducing the mass of metal around the connection of joints. It is very important to correctly locate the collars or joints so that the depth or location of a tool in the cased well can be determined. Given the fact that casing joints have uniform spacing, the depth of a particular tool suspended in the well can be determined if the casing collars or joints can be correctly counted.
With the advent of the improved threaded joints which reduces the amount of metal at a joint connection, detection is more difficult. Such casing joints are described as having flush collar joints. The adjacent joints are assembled by pin and box construction. This defines a relatively small threaded area which is much more difficult t o locate.
Magnetic casing collar locators of the past have been able to tolerate a wide range of magnetic field dispersion. They have been able to tolerate degraded signal to noise ratios. However, with the advent of the improved flush collar construction, magnetic focusing becomes more crucial. One device used heretofore is described in U.S. Pat. No. 3,434,046. In that disclosure, a common magnetic core for coils is used, the core being well known in the transformer art as a stack of E-shaped laminations which are assembled to form the core. Such a device, however, operates best in a decentralized mode, ideally urged against the wall of the casing. There is frictional drag in such a device. Therefore, its rate of travel in the cased hole is limited. Moreover this particular patent shows centralizing springs which drag against the casing and which ultimately wear out as a result of the dragging action during use. Sensitivity to aberrations in the metal wall is also increased. For instance, if such a detecting structure moves directly over pits, mill scale, etc., the apparatus may very well describe such common metal imperfections as a collar. A centralized tool has the advantage of providing a relatively smaller response to pits because they typically do not fully encircle the casing in the same fashion as does a set of threads. A centralized tool thus is completely surrounded by the threads and perturbations are observed fully around such a tool in the magnetic field which is directed by the improved magnetic system of this invention.
This invention utilizes three permanent magnets which are axially aligned along the tool axis with opposing poles abutted. The magnets are aligned axially as the tool is centered in the casing. The magnets therefore are ideally moved along the centerline axis of the casing. The tool supports the three magnets so that the opposing poles form a focused toroidally shaped magnetic field radiating radially outwardly from the center magnet. Thus, the focused magnetic field has an axial extent that is approximately equal to the length of the center magnet. Ideally, the magnets are identical in magnetic field strength and have identical physical dimensions. This system thus comprises a focused magnetic field system which utilizes permanent magnets to define a toroidally shaped magnetic field sensitive to the threads in the casing wall as a discontinuity in its magnetic circuit. The magnets support detector coils thereon. The preferred embodiment uses two identical detector coils which are spaced along the length of the three serially arranged permanent magnets. As a disturbance or magnetic field anomaly is encountered in the magnetic field, it is observed as a voltage signal induced first in one coil and subsequently as an opposite polarity voltage signaling the other coil. The two coil output voltage signals are provided as inputs to a differential amplifier set up as a summing amplifier so that the two opposing signals cancel one another. This cancels various extraneous signals as will be described, and enables the two summed signals to describe the passage of the focused magnetic field by a set of threads by signal deflection. In other words, a casing collar threaded section is located and identified by means of a strong voltage signal from the summing amplifier which deflects first positive and then negative (or in the reverse sequence depending on the direction of travel of the tool). The voltage signal is then provided to parallel first and second low pass filters. The filtered signal outputs are then provided to a differential amplifier where the two signals are summed. This again helps eliminate noise in the signals and provides a pass band which is defined by the separate cutoff points of the two filters. Useful information is obtained in a specified pass band and can thereafter be additionally amplified and applied to a recorder wherein casing collars can then be counted.